The spawn chance of chubby enemies is very low, whenever a badass enemy is spawned it has a rare chance of spawning as a chubby. This makes them very difficult to find in the wild, especially at lower levels and play modes.

Spawn Strategy

Due to their spawn nature chubbies are more frequently encountered in areas that have a higher chance of spawning badasses and among enemies that are more frequently found as badasses, most notably varkids. Other factors such as the playthrough mode and the number of players in the game also work to increase the rate of badasses spawning, thus increasing the rate of chubbies spawning. A 4 player game will be much more likely to see a chubby enemy than a single player game.

Places that more easily spawn chubbies:

Caustic CavernsDue to the nature of varkid evolution, it is easier to spawn chubby enemies here than in other places. Also of note are the spiderant colonies. Together these make the Caustic Caverns one of the best places to farm chubbies in a game without DLCs. The areas where enemies spawn are also quite conveniently isolated and segregated, which allows players to farm without other enemies attacking their spawns. Chubby farming here couples well with farming for Vermivorous the Invincible

Tundra ExpressThis is another area where chubby farming is made easier due to the presence of varkid colonies. Although this area has other enemies that will target Vault Hunters and interfere with farming.

The DustIn this area there is Bug Gulch behind Ellie's Garage. This area has a large number of spiderants, with very little resistance from other enemy types. Farming this area is best coupled with farming The Black Queen, Gettle, and Mobley.

Natural Selection Annex(DLC only)During the Creature Slaughter missions, badasses have a much higher rate of spawning, though in order to farm this area effectively one of the higher rounds will need to be deliberately failed, preferably on the final wave after the possibility of additional chubbies has been exhausted.

Arid Nexus - BadlandsThere are ~12 skag dens on the map. The quest Hungry Like the Skag will highlight the spawn areas on your map. Each den area is able to spawn Chubbies and Badasses. Nearby the Fast Travel station there are 6 you can check quickly—3 on the ridge just right of the player spawn, 2 near a house area just ahead over a small ledge, and 1 by the skag pool straight ahead of the Fast Travel station. Utilizing the Save/Quit strategy, this area yields fast and reliable results in single player.

Three Horns - ValleyIf using the Fast Travel station there are a few skags on the way to fight Doc Mercy. If none of these enemies are tubby skags then you can [Save]/[Quit], and enter the game again to check for the tubbies.

Hallowed Hollow (DLC only) The colony and last the infested forge spawn Spiderants workers and above more commonly than spiderlings, badasses have a higher chance of spawning resulting in more chubby Spiderants than in other locations. Using the save and quit strategy will yield reliable results.